Process for the liquefaction and subcooling of natural gas

ABSTRACT

A process for the liquefaction and subcooling of natural gas with a Claude closed refrigerating cycle comprising compressing gaseous cycle medium; cooling resultant compressed gaseous cycle medium; dividing cooled compressed gas into two streams; engineexpanding one stream; and cooling the other stream with the engine-expanded stream to such an extent that said other stream becomes partially liquefied after a subsequent throttle expansion thereof; the improvement comprising employing as the cycle medium, a mixture of nitrogen and methane.

United States Patent [191 Etzbach et al.

[ PROCESS FOR THE LIQUEFACTION AND SUBCOOLING OF NATURAL GAS [75]Inventors: Volker Etzbach, Munich; Wolfgang Forg, Grunwald;-Peter Grimm,Munich, all of Germany [73] Assignee: Linde Aktiengesellschaft,

' Wiesbaden, Germany [22] Filed: Mar. 6, 1972 [21] App]. No.: 231,984

[30] Foreign Application Priority Data June 25, 1974 3,616,652 11/1971Engel 62/11 3,677,019 8/1969 Olszewski 62/9 FOREIGN PATENTS ORAPPLICATIONS 635,337 l/l962 Canada 62/9 OTHER PUBLICATIONS Kleemenko,One Flow Cascade Cycle, Progress in Refrigeration Science andTechnology, Vol. 1 (1960).

Primary Examiner-Norman Yudkoff Assistant Examiner-Arthur F. PurcellAttorney, Agent, or Firm-Millen, Raptes & White 5 7] ABSTRACT A processfor the liquefaction and subcooling of natural gas with a Claude closedrefrigerating cycle comprising compressing gaseous cycle medium; coolingresultant compressed-gaseous cycle medium; dividing cooled compressedgas into two streams; engineexpanding one stream; and cooling the otherstream with the engine-expanded stream to such an extent that said otherstream becomes partially liquefied after a subsequent throttle expansionthereof; the improvement comprising employing as the cycle medium, a

mixture of nitrogen and methane.

13 Claims, 3 Drawing Figures PATENTEDJUNZSW 3.818.714

swan 2 or 2 I FIG'.3

BACKGROUND OF THE INVENTION This invention relates to a process forliquefaction and subcooling of natural gas by a Claude cycle. This cyclecomprises compressing cycle gas; cooling same; dividing cooledcompressed gas into two streams; engine-expanding one stream; andcooling the other stream with the engine-expanded stream to such anextent that said other stream becomes partially liquefied after asubsequent throttle expansion thereof. By the evaporation of thisresultant liquid, peak cold is made available, namely that amount ofrefrigeration required for subcooling the natural gas. Beforesubcooling, the natural gas has already been liquefied under a higherpressure by heat exchange with the cycle gas, but the subcooling ensuresthat the natural gas remains practically entirely in the liquid phaseeven after expansion to the pressure of the storage tank. If it isdesired to avoid operation of the Claude cycle under a negativepressure, it is essential to employ as the cycle medium a gas having alower boiling point than methane, nitrogen being conventionallyemployed.

One disadvantage of the above-described process is that the liquid cyclenitrogen is evaporated isothermally, i.e., yields cold at a constanttemperature, but the liquid natural gas to be subcooled can absorb thisrefrigeration only at a decreasing temperature. Consequently, because ofthis temperature gradient, the refrigeration is transferred at atemperature level lower than that necessary for cooling purposes. Thus,in the peak-cold generator, the occurrence of large heat transfertemperature differences AT is unavoidable, increasing the thermodynamicirreversibility and energy requirements of the process.

Another disadvantage is to be seen from the fact that the enthalpygradient in the expansion turbine, as well as the J oule-Thomson effectin the peak-cold generator are relatively small in case of nitrogen, sothat a large amount of gas must bypass the turbine and enter through thethrottle valve, and therefore cannot be utilized for the production ofcold by engine expansion. For this reason, in addition to the fact thatthe isothermal Joule-Thomson effect of the nitrogen is also small at thewarm end, the specific refrigeration capacity of the cycle per Nm ofcirculated gas is relatively low.

Finally, another disadvantage is that it is necessary, in order toprovide makeup nitrogen due to cycle losses, either to keep purenitrogen in readiness or to separate same continuously from the naturalgas. The latter expedient involves a substantial plant investment,however, in case the natural gas is low in nitrogen.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a process for the liquefaction of natural gas by the use of aClaude closed refrigerating cycle wherein one or more of the followingadvantages are obtained: a lower energy requirement, a higherrefrigerating capacity per unit quantity of the cycle gas, and easieravailability and lower cost of makeup fluid required to effect leakagelosses.

Upon further study of the specification and appended claims, otherobjects and advantages of the present invention will become apparent.

2 These objects are attained, according to this invention, by using amixture of nitrogen and methane as the cycle medium.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a boiling point-compositiondiagram for N -CH at various pressures; and

FIGS. 2 and 3 are schematic views of preferred embodiments of theprocess.

DETAILED DISCUSSION One advantage of this process is that theevaporation of the liquid methane-nitrogen mixture does not take placeat a constant temperature, but rather at a sliding temperature inaccordance with the boiling point diagram of nitrogenmethane at theevaporation pressure under consideration, each evaporation temperaturebeing associated with a specific mixture (compare FIG. 1). Therefore, bya selection of the evaporation pressure and the composition of the cyclegas, the temperature range of the evaporation can be very well adaptedto the temperature range of the subcooling. Thus, the temperaturedifferences in the peak cooler are small and the energy losses causedthereby are minor. Since the methane, in mixture with the nitrogen, isvaporized at its partial pressure, i.e., a pressure lower than theambient evaporation pressure, it is possible to obtain, with methane, aspecific low temperature at a relatively high pressure level.

The refrigerating capacity of the Claude cycle is likewise improved bythe addition of methane to the nitrogen. For the methane, as a less thanideal gas, increases the Joule-Thomson effect in the peak cooler, sothat the amount of cycle gas to be fed to the J-T valve can be smaller,and a larger proportion of the cycle gas can be fed to the engineexpansion. Additionally, the enthalpy gradient in the turbine and thusthe specific refrigerating capacity of the cycle, based on the unitquantity of circulated gas, is larger in the case of methane than in thecase of nitrogen. The same holds true for the isothermal Joule-Thomsoneffect at the warm end. Therefore, in total, the amount of cycle gasrequired to produce a specific amount of cold is reduced.

Finally, the use of a methane-nitrogen mixture as the cycle gas affordsthe further advantage that the leakage losses of the cycle medium can becompensated for at less cost by utilizing natural gas therefor. This isso because the separation of a gas rich in nitrogen prior to theliquefaction is in most cases absolutely necessary, even in case ofgases of low nitrogen content, since too great a drop in theliquefaction temperature of the natural gas must be avoided. Theabove-described advantage is especially noticeable in the processing ofnatural gases having a low nitrogen content, for otherwise rectificationdevices would have to be provided in this pro cedure wherein thenitrogen is separated from the natural gas not only in a high purity,but also in good yields. The process according to the present inventionoffers advantages even if the natural gas contains no nitrogen at all,since only that proportion of the leakage losses associated with thenitrogen need be made up from a source externally of the plant, whereasthe lost methane can readily be taken from the natural gas.

In the determination of the specific quantitative ratio of nitrogen:methane to be employed, the following criteria must be observed: As setforth hereinabove, the

addition of methane to the nitrogen results in an improvement of thespecific refrigerating capacity of the cycle gas; therefore, as high amethane content as possible would be desirable, all other things beingequal. However, it can be seen from FIG. 1 that, at a constant pressure,the boiling point temperature of a mixture in the region of highnitrogen concentrations (down to a concentration of about 30-40 percentof nitrogen equal to 60-70 mol percent CH is elevated to a relativelyminor extent by the addition of a specific quantity of methane, e.g., amol percent, increment in the liquid, but that the addition of the samequantity of methane in a zone of lower nitrogen concentrations (e.g.below 30 mol percent nitrogen equal to above 70% CH causes a substantialelevation in the boiling temperature. These relationships have asignificant effect on the suction intake pressure of the compressor aswill now be explained with reference to numerical values derived fromFIG. 1.

In order to maintain a boiling temperature of 1 10 K, a pressure of 16ata. (atmospheres absolute) is required in the case of pure nitrogen(point A), and in the case of a mixture containing 55 percent ofmethane, a boiling pressure of 8 ata. is necessary (point B). Thus, theaddition of 55 percent of methane only effects a lowering of the boilingpressure to one-half the value. In contrast thereto, in the region ofhigh methane concentrations, the boiling point pressure is lowered bythe factor of one-half already with a substantially lower quantity ofadded methane. For example, if the methane concentration is increasedonly 10 percent from 85 percent (point C) to 95 percent (point D), theboiling point pressure at 1 10 K is lowered from about 4 ata. to about 2ata. Therefore, in the zone of high methane concentrations, an increasein the refrigeration capacity of the cycle by the further addition ofmethane can be obtained only at the cost of a considerablepercentagewise drop in the suction pressure; consequently, in thisregion of high methane concentration, the number of required compressorstages increases sharply.

In accordance with a preferred embodiment of the invention, therefore,the nitrogen concentration of the cycle gas is, on a molar basis, atleast 20 percent, preferably at least 40 percent, conversely, themaximum preferred nitrogen concentration is 80 percent, particularly 60percent.

To provide makeup for leakage losses according to this invention, anitrogen separation unit is incorporated in the natural gas liquefactionprocess. From the head of this unit there is withdrawn a fraction havinga nitrogen concentration of at least the same level as that of the cyclegas, and this fraction is then fed into the cycle as makeup gas. If thisfraction contains more nitrogen than the cycle gas, then supplementalmethane from the purified natural gas (freed of CO H 0, and heavyhydrocarbons) can be added thereto.

One negative feature of the cycle of this invention is that the cyclegas must be precooled to a very low temperature prior to entering theexpansion engine so that it can be cooled to a sufficiently lowtemperature during the expansion. This precooling can be conductedconventionally with a multistage refrigerating machine operating withfreon, ammonia, or propane, also including, in many cases, an essentialthird stage of vacuum. These refrigerating machines exhibit the samedisadvantage described above in connection with the Claude cycle, namelythat the refrigeration is transferred at a constant temperature, to astream having a sliding temperature. Thus, unnecessarily largetemperature differences inherently must occur in the heat exchangersassociated with the individual pressure stages. To avoid thesedisadvantages, according to a further embodiment of this invention,there is employed a mixture of methane, propane, and optionally ethaneas the cycle medium for the precooling cycle.

A main advantage of this latter feature is that the refrigerationliberated during the evaporation is transferred at a decreasingtemperature so that small temperature differences (ATs) can be employedin the heat exchangers, resalting in a thermodynamically more efficientprocess. Since the propane and any ethane present evaporate under apartial pressure lower than the total pressure ambient during theevaporation, the desired low temperature is obtained at a higher totalpressure than would be the case when evaporating pure ethane or propane.In other words, because the transfer of peak cold involves a smallerpressure drop, the number of compressor stages and thus the number ofevaporators is decreased, and the cost of control elements is alsoreduced. Finally, it is to be noted that leakage losses amounting toabout 1 5 parts per thousand of the quantity of cycle medium canfrequently be covered, in the case of methane and ethane, merely bysimple separation from the natural gas proper, i.e., they need not bestored in additional tanks. Propane is always available in any case sothat the BTU value of the gas can be adjusted or desired. The proportionof each of methane, propane and optionally ethane in the total of thecycle gas is about 20-50 molar percent, respectively. If a very lowprecooling temperature is to be reached, e.g., 200 to 215 K, there isemployed a cycle gas having a composition, of in moles percent 40 and 60methane; 25 and 60 percent propane; and 0 and 15 ethane.

The above-described advantages are of special importance when theprecooling cycle is operated, according to a further preferredembodiment of the invention, in a single stage, i.e., when theevaporation of the compressed, cooled, and throttle-expanded refrigeranttakes place at a uniform pressure level set by the compressor suctionpressure. In this way, for example, a precooling temperature of 60 C.can be attained in a single stage with a mixture consisting ofapproximately equal parts of methane and propane, whereas, in contrast,a three-stage plant would be required for this purpose if freon wereemployed as the refrigerant. The precooling temperature attainable inthis manner is generally sufficient, even in case of low natural gaspressures, to result in the condensation of the heavy hydrocarbonswhich, otherwise, could lead to obstructions in the low-temperaturesections. Thus, the plant can be rapidly brought to the required coldoperating condition.

By utilizing the above-described single-stage precooling cycle, an evenlower precooling temperature, e.g. to 200 K, according to a stillfurther preferred embodiment of this invention, as follows: a fractionof the cycle medium remains in the gaseous phase downstream of the finalcooler of the compressor, and the fraction containing lower boilingcomponents is separated from the liquid fraction containingpredominantly the higher-boiling hydrocarbons. Both separated gas andliquid are then cooled by heat exchange with the liquid evaporating atthe suction pressure of the compressor. The gas rich in lower-boilingcomponents is thus totally condensed, and is then in the liquid phaseexpanded (pressurereduced) to the suction pressure of the compressor.The resultant liquid is then evaporated to effect said totalcondensation.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIGS. 2 and 3, thenatural gas to be processed (0.6,780 Nm /hr.), after having been freedof water, carbon dioxide, and hydrogen sulfide, and having approximatelythe following composition: 2 percent nitrogen, 94 percent methane, 3percent ethane, l percent propane and higher hydrocarbons, is fed to theplant, via conduit 1, at 298 K and under 39 ata. and is then cooled inthe heat exchanger 2 to 216 K. During this step, the C and higherhydrocarbons are substantially condensed, which would otherwise causeclogging in subsequent sections of the plant. The liquid is thusseparated from the gaseous phase in phase separator 3, evaporated andwarmed in heat exchanger 2, and discharged from the plant via conduit 4.The portion remaining in the gaseous phase is further cooled in heatexchanger 5 to about 190 K; during this step, a liquid is obtainedconsisting of about 85 percent methane, percent ethane, and 5 percentpropane. This liquid after being passed in separator-collector 6, isbranched into two streams, and the amount necessary to make up forleakage losses of the precooling cycle is introduced into the latter viaconduit 7. The remainder is evaporated and warmed in heat exchangers 5and 2, and then discharged from the plant via conduit 4.

A portion of the gas withdrawn from the separator 6 overhead is nowfurther cooled in the heat exchanger 8 to 163 K and expanded into thenitrogen separation column 9 operating at 22 ata. The remaining gas isconducted through a heating coil located in the sump of the column 9 andis then expanded into the midsection of column 9. At the head of thecolumn 9, a temperature of about 150 K is maintained; the gaseous headproduct consists of 50 percent of a methane and 50% of nitrogen. Thishead product is discharged from the plant via conduit 4, except for thatamount of gas required for providing makeup due to the leakage losses ofthe Claude refrigerating cycle and which is fed into said cycle viaconduit 10. From the sump of the column 9, 5,820 Nm /h. of liquidnatural gas is withdrawn having a temperature of 167 K and approximatelythe following composition: 97 percent methane, 1 percent nitrogen, and 2percent ethane. This liquid is passed to the heat exchangers l1 and 12and cooled therein to l 1 1 K so that, during the subsequent expansionin the valve 13 to the pressure of the storage tank (slightly above 1ata.), only a minimum amount of liquid is evaporated (about 40 Nm lh).

The refrigeration required for the liquefaction is provided by a Claudecycle with precooling by a one-stage cycle based on a mixture of gases.This mixture comprises 50 percent methane and 50 percent nitrogen and isemployed as the cycle medium. This gas (37,900 Nm lhr) is compressed, incompressor 14, to 25.5 ata., and after being cooled, is furthercompressed to 35.5 ata. in compressor 15. The gas enters the heatexchanger 2 at a temperature of 298 K and is precoolcd therein and alsoin the heat exchanger 5 to 197 K.

, 35,600 Nm /h. of cycle gas is then expanded in the expansion turbine16 to 8 ata. and, during this step, is cooled to 138 K. A portion ofthis gas is branched off via conduit 17 and serves for cooling the headof column 9; the main quantity is fed, via conduit 18, to the cold endof the heat exchanger 11, heated therein and in heat exchangers 8, 5,and 2, to ambient temperature and thereafter recompressed in thecompressor 14.

The proportion of the cycle medium not subjected to engine expansion,i.e., 2,300 Nm /h., is cooled in conduit 19 under its pressure of 35.5ata. in heat exchangers 8, l1, and 12, to 111 K. During the subsequentthrottle expansion to 8 ata. in valve 20, the temperature drops to 109K, so that the liquid natural gas can be subcooled to l 1 1 K by heatexchange with the boiling cycle liquid, before it is expanded in valve13. At 21, the throttle-expanded cycle medium is combined with theengine-expanded cycle medium, and is warmed and recompressed togethertherewith.

The cycle medium of the precooling cycle consists of 45 percent methane,5 percent ethane, and 50 percent propane. 4,200 Nm lh. of this gas iscompressed in compressor 22 from 10 ata. to 50 ata., cooled andsimultaneously liquefied in heat exchanger 2, and then expanded to 10ata. in valve 23. The precooling temperature attainable in this manner,i.e., the temperature at which the gaseous streams to be cooled leavethe cold end of the heat exchanger 2, is 216 K. The evaporated andwarmed cycle medium is then recompressed in the compressor 22. Theleakage losses of the cycle, as mentioned above, are, in part,compensated for by the liquid withdrawn from separator 6 via conduit 7.Since this liquid contains less propane than the cycle medium, purepropane or a gas more enriched therewith must be added. This can be doneby conducting the gaseous stream returning to the compressor 22 viaconduit 30 through the tank 28 filled with liquid propane, rather thanvia conduit 29. The dome 31 serves as an entrainment separator for theseparation of droplets of liquid propane.

If the natural gas is available at a lower pressure, then a lowerprecooling temperature is required. In order to attain such lowertemperature, the precooling cycle shown in FIG. 3 is employed. The cyclemedium consists of about percent methane, 5 percent ethane, and 25propane. The liquid formed in the secondary cooler of the compressordenoted by 24 is separated from the gaseous phase in the separator 25,cooled in heat exchanger 2, expanded in valve 23' from 35 to 8 ata., andreevaporated and warmed in heat exchanger 2. The gaseous phase enrichedin the lower-boiling components of the cycle medium, is withdrawn fromseparator 25, conducted, via conduit 26, through the heat exchangers 2and 5, being cooled and liquefied during this step, and then expanded invalve 27 from 35 to 8 ata. By the evaporation of the resultantpressurereduced liquid, a precooling temperature of about K is attacinedat the cold end of the heat exchanger 5. The cycle medium evaporated andwarmed in heat exchanger 5 is recombined with the cycle medium expandedin valve 23', and the combined stream is then recycled to the suctionside of compressor 22.

For the sake of clarity, B in FIG. 3 denotes the sum total of theremaining gaseous streams to be cooled, i.e., the natural gas to beliquefied and the compressed cycle medium of the Claude cycle; and Cdenotes the sum total of the other gaseous streams to be warmed, i.e.,the fractions obtained during the natural gas liquefaction and to bedischarged from the plant in the gaseous phase, and the expanded cyclemedium of the Claude cycle.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions. Consequently, such changes and modifications are properly,equitably, and intended to be within the full range of equivalence ofthe following claims.

What is claimed is:

1. In a process for the liquefaction and subcooling of natural gas witha Claude closed refrigerating cycle comprising compressing gaseous cyclemedium; dividing cooled compressed gas into two streams; engineexpandingone stream; cooling the other stream with the engine-expanded stream tosuch an extent that said other stream becomes partially liquefied aftera subsequent throttle expansion thereof and passing resultant partiallyliquefied stream in indirect heat exchange relationship with liquidnatural gas to subcool the liquid natural gas so that it remainssubstantially in the liquid phase after being expanded to the pressureof a storage tank;

the improvement comprising employing as the cycle medium, a mixture ofnitrogen and methane.

2. A process according to claim 1, wherein the nitrogen molarconcentration in the cycle medium is at least 20 percent.

3. A process according to claim 1, wherein the nitrogen molarconcentration in the cycle medium is at least 40 percent.

4. A process according to claim 1 further comprising separating thenatural gas from CO H and heavy hydrocarbons; separating a nitrogenenriched stream from resultant purified natural gas, the nitrogencontent of said enriched stream being at least as large as that of saidcycle medium and feeding said nitrogen enriched stream into the Claudecycle in sufficient amounts to provide makeup cycle medium.

5. A process according to claim 1 further comprising precooling said onestream prior to engine expanding thereof with a refrigeration precoolingcycle based on a cycle medium comprising a mixture of methane andpropane ethane.

6. A process according to claim 5, wherein said precooling refrigerationcycle is conducted in a single stage.

7. A process according to claim 5, said precooling refrigeration cyclecomprising a precooling compressor and a cooler downstream of theprecooling compressor, and further comprising with drawing a mixture ofgas and liquid from said cooler, said liquid containing predominantlyhigher-boiling hydrocarbons and said gas being rich in lower-boilingcomponents; separating the gas and the liquid; said separated gas andliquid streams in heat exchange with liquid evaporating at the suctionpressure of the compressor, totally condensing said gas rich inlower-boiling components, the resultant liquid being expanded to thesuction pressure of the compressor, whereby the total condensation iseffected by the evaporation of the thus-formed, expanded liquid.

8. A process according to claim 6, said precooling refrigeration cyclecomprising a precooling compressor and a cooler downstream of theprecooling compressor, and further comprising with drawing a mixture ofgas and liquid from said cooler, said liquid containing predominantlyhigher-boiling hydrocarbons and said gas being rich in lower-boilingcomponents; separating the gas and the liquid; said separated gas andliquid streams in heat exchange with liquid evaporating at the suctionpressure of the compressor, totally condensing said gas rich inlower-boiling components, the resultant liquid being expanded to thesuctionpressure of the compressor, whereby the total condensation iseffected by the evaporation of the thus-formed, expanded liquid.

9. A process as defined by claim 5 wherein said precooling cycle mediumfurther comprises ethane.

10. A process as defined by claim 2 wherein the maximum molarconcentration in the cycle medium is nitrogen.

11. A process as defined by claim 2 wherein the maximum molarconcentration in the medium is 60 percent nitrogen.

12. A process as defined in claim 3 wherein the maximum molarconcentration in the cycle medium is 80 percent nitrogen.

13. A process as defined by claim 3 wherein the maximum molarconcentration in the medium is 60 percent I UNITED STATES PATENT OFFICE7 CERTIFICATE :CQRRECTION Patent NO. Dated June 25! Inventor(s) VolkerEtzbach, et a1.

It is certified that error appears in the above-.iderlti-fiedpatent 1and that said Letters Patent are hereby corrected as shown below:

IN THE cLAIMsi CLAIM 5, COLUMN 7, LAST LINE OF THE CLAIM:

Delete "ethane".

Signed and sealed this 24th day of September 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents 7 uscoMM-bc OO376-P69 fi U.S GOVERNMENT PR NTING OFFICE: 19,0-356-33,

: ORM PO-1050 (10-69)

2. A process according to claim 1, wherein the nitrogen molarconcentration in the cycle medium is at least 20 percent.
 3. A processaccording to claim 1, wherein the nitrogen molar concentration in thecycle medium is at least 40 percent.
 4. A process according to claim 1further comprising separating the natural gas from CO2, H2O and heavyhydrocarbons; separating a nitrogen enriched stream from resultantpurified natural gas, the nitrogen content of said enriched stream beingat least as large as that of said cycle medium and feeding said nitrogenenriched stream into the Claude cycle in sufficient amounts to providemakeup cycle medium.
 5. A process according to claim 1 furthercomprising precooling said one stream prior to engine expanding thereofwith a refrigeration precooling cycle based on a cycle medium comprisinga mixture of methane and propane ethane.
 6. A process according to claim5, wherein said precooling refrigeration cycle is conducted in a singlestage.
 7. A process according to claim 5, said precooling refrigerationcycle comprising a precooling compressor and a cooler downstream of theprecooling compressor, and further comprising with drawing a mixture ofgas and liquid from said cooler, said liquid containing predominantlyhigher-boiling hydrocarbons and said gas being rich in lower-boilingcomponents; separating the gas and the liquid; said separated gas andliquid streams in heat exchange with liquid evaporating at the suctionpressure of the compressor, totally condensing said gas rich inlower-boiling components, the resultant liquid being expanded to thesuction pressure of the compressor, whereby the total condensation iseffected by the evaporation of the thus-formed, expanded liquid.
 8. Aprocess according to claim 6, said precooling refrigeration cyclecomprising a precooling compressor and a cooler downstream of theprecooling compressor, and further comprising with drawing a mixture ofgas and liquid from said cooler, said liquid containing predominantlyhigher-boiling hydrocarbons and said gas being rich in lower-boilingcomponents; separating the gas and the liquid; said separated gas andliquid streams in heat exchange with liquid evaporating at the suctionpressure of the compressor, totally condensing said gas rich inlower-boiling components, the resultant liquid being expanded to thesuction pressure of the compressor, whereby the total condensation iseffected by the evaporation of the thus-formed, expanded liquid.
 9. Aprocess as defined by claim 5 wherein said precooling cycle mediumfurther comprises ethane.
 10. A process as defined by claim 2 whereinthe maximum molar concentration in the cycle medium is 80% nitrogen. 11.A process as defined by claim 2 wherein the maximum molar concentrationin the medium is 60 percent nitrogen.
 12. A process as defined in claim3 wherein the maximum molar concentration in the cycle medium is 80percent nitrogen.
 13. A process as defined by claim 3 wherein themaximum molar concentration in the medium is 60 percent nitrogen.